A plan to destroy the world's last known samples of smallpox virus later this year is threatened by the growing suspicion that secret caches of the microbe probably exist, increasing the chances it could fall into the hands of a rogue nation or terrorist organization.

If that is the case, some scientists believe, stocks of the deadly virus should be kept so they can be used to help develop antiviral drugs and a better vaccine against the disease, which was eradicated from the world in 1978. That view, however, is not universal, with some people saying that destruction of the known viral stocks would discourage the use of any pirated ones.

Today, an expert panel convened by the National Academy of Sciences Institute of Medicine will offer its opinion of the future scientific needs for the virus, which officially exists only at the federal Centers for Disease Control and Prevention in Atlanta and at a laboratory run by the Russian government in Siberia.

The report was requested by the departments of Defense, Energy and Health and Human Services, and is expected to carry substantial weight in the debate within the Clinton administration on whether to support the World Health Organization's recommendation to incinerate the smallpox stocks on June 30.

That plan will be reviewed in May, when representatives of WHO's 190 member countries meet in Geneva. The destruction date was set by consensus at a similar meeting in 1996. In the intervening three years, scientists in the United States and Russia cloned the virus's genes into harmless samples suitable for research and performed several other studies. Although some scientists argued for keeping the virus indefinitely, the consensus was that this posed risks far outweighing any insights that might be gained in the study of a virus nobody in the world could ever contract again.

Events of the last few years, however, have challenged that last assumption.

"One would have to be ridiculously optimistic to conclude there are now only two locations in the world where smallpox is stored. And I do mean ridiculously optimistic," Amy E. Smithson, an expert on biological and chemical weapons proliferation at the Henry L. Stimson Center in Washington, said last week.

This view is shared, somewhat less emphatically, by advocates for virus destruction, who until recently doubted there were secret stores of smallpox.

"I think there's more in Russia than in the one center [designated as a smallpox repository]," said Donald A. Henderson, the American physician who led the global smallpox eradication effort from 1966 to 1977. "There's no question about that."

The debate has been altered by recent revelations that the Soviet Union made industrial quantities of smallpox for years after it signed a 1972 treaty prohibiting such work. Russian credibility has suffered hugely.

"I think the likelihood that the Russians destroyed everything except what they had in the WHO laboratory is very small," said Frank Fenner, an Australian physician, now 84, who chaired the global commission that finally "certified" the world as smallpox-free in 1980.

The question of whether the world must retain the virus to plan against its possible use as a biological weapon is the subject of intense scientific debate.

"If we are serious about bio-defense, the stocks are necessary for developing an antiviral drug, and possibly necessary for developing a vaccine more suitable for the general population than the current one," said Alan Zelicoff, a scientist at Sandia National Laboratories in New Mexico and a consultant to the Defense Department.

Proponents of destruction don't deny that smallpox's potential use as a biological weapon is worrisome. They argue, however, that the essential scientific work can be done without live samples of the virus, and that destruction will make a moral statement about its use that even terrorists could not ignore.

If smallpox were to reappear, Fenner said, "it couldn't be the result of untoward escape from a laboratory. It would have to be from deliberate use. That could then be condemned as a horrendous crime against humanity – reintroducing a disease that the world with great effort had freed itself from," Fenner said.

Last year, WHO polled its 190 member countries to learn whether there was still consensus for destroying the known stocks of virus this summer. About 70 nations answered. The United States, Britain, France and Italy said they were undecided. Russia said the virus should be retained. All the rest favored destruction.

The World Health Assembly, WHO's governing body, has no power to enforce its recommendations beyond persuasion. Consequently, it prefers to operate by unanimous consent. Several people familiar with the assembly say it's unlikely to pass a resolution recommending destruction of smallpox stocks if both the United States and Russia oppose the idea.

Smallpox, which afflicted humanity for 10,000 years, was supposed to be a scourge nobody needed to worry about anymore.

Variola virus – the formal name for smallpox virus – killed untold millions of people over the centuries. Like chickenpox, it is highly contagious and produces a striking, pustular rash. Unlike that infection, however, it kills about 20 percent of people who contract it.

The last naturally occurring case of the infection appeared in Merca, Somalia, in October 1977. The last cases ever were in Birmingham, England, in 1978, when virus apparently escaped into the duct work of a laboratory. One person died, and the scientist in charge of the laboratory committed suicide.

Smallpox vaccine prevents infection in most cases, even if given within a few days after a person has had contact with the virus. The protection isn't lifelong, however. Routine vaccination in the United States ended in 1971. Except for some soldiers and laboratory workers, nobody has been vaccinated since 1983. Today, virtually the entire population of the globe is susceptible to the disease.

In the early 1980s, most samples of smallpox virus in labs around the world were destroyed. Samples of about 400 strains were gathered at the CDC repository, and about 120 strains at a scientific institute in Moscow. The Russian collection was later moved to a laboratory called VECTOR in the Siberian city of Koltsovo, previously a site of the Soviet biological weapons program.

The idea that smallpox posed a real threat took hold a year ago, when Ken Alibek, a former high official at VECTOR who had defected to the United States in 1992, revealed the extent of the Soviet bioweapons program.

Alibek testified to Congress in May that through the end of the 1980s, the Soviet Union had produced "hundreds of tons of anthrax weapon . . . along with dozens of tons of smallpox and plague." Furthermore, scientists had worked on splicing genes from other pathogens into smallpox virus to produce novel, and possibly vaccine-evading, microbes. He added that he was "convinced" Russia's biological weapons program "has not been completely dismantled."

Alibek, who lives in Northern Virginia, is the only public source for most of the information he revealed. Nevertheless, his statements are considered credible by many experts, and some with access to intelligence sources say they can be corroborated.

"This country owes him an extreme debt of gratitude. His information was certainly eye-opening," said William C. Patrick, the 73-year-old former head of the product development division of the Biological Warfare Laboratories at Fort Detrick in Frederick. (American work on offensive biological weapons ended in 1969.)

Whether smallpox exists anywhere outside Russia or the United States is a subject of much speculation. There is no hard public evidence that it does. No smallpox was found in Iraq by United Nations weapons inspectors. However, the inspectors were told by one former Iraqi bioweapons researcher that in 1990 some of his colleagues had worked on camelpox.

Camelpox is genetically close to smallpox, but does not infect humans. Some people have inferred that Iraq had smallpox, and was using camelpox as a safe "surrogate" in tests.

There are even remoter possibilities that smallpox remains in recoverable, if unintentional, repositories.

At a scientific meeting in Munich last year, Russian researchers reported that in 1991 they tried to extract virus from the body of 19th century smallpox victim unearthed from a frozen grave in Yakutia. Although they were unsuccessful, antibodies against smallpox reacted with the extracted tissue, suggesting remnants of the virus remained.

Even if a consensus emerges that research on live smallpox virus is warranted, the obstacles are huge.

The United States now has only one site where such work is permitted – the "Biosafety Level 4" laboratories at the CDC, where scientists wear protective suits when they work with the virus. In the last 15 years, researchers there have grown live smallpox virus fewer than a half-dozen times. The single request from non-American scientists to work with smallpox was denied.

Fenner, the Australian veteran of the eradication campaign, believes this would likely be the norm, even if the world decides to hold onto smallpox.

"I think the stores would be left in their liquid nitrogen containers, and would have to be guarded by both microbiological and military security forever."